Groups API: Only allow merge requests to be merged when all discussions are resolved

Proposal

Back in Add setting to only allow merge requests to be ... (gitlab-foss!6385 - closed) we created a project-level setting: only_allow_merge_if_all_discussions_are_resolved (Set whether merge requests can only be merged when all the discussions are resolved.).

This issue is to propose we duplicate this setting, but at a group-level where it would apply for all nested projects. At the API Level whereby one can configure this setting, and check its status via the Rest API.

API example

curl -H "PRIVATE-TOKEN: $token" "https://gitlab.example.com/my-group" -s | jq"

{
...
  "only_allow_merge_if_all_discussions_are_resolved": true,
...

Existing capability

Within the UI, for any group, one can go to: Settings -> General -> Merge Requests and see the setting: All threads must be resolved. This is the same setting we're proposing, but at the API level.

screenshot